Service interval monitors for the automotive industry are relatively new and are used to optimally service motor oil, filters and other replacement parts on vehicles.
Such preventive service has historically been performed on an elapsed time or elapsed mileage basis. More recent cost-benefit analyses have shown, however, that such preventive maintenance based solely on elapsed time or mileage is not a good indicator of actual service requirements. A far better indicator of service needs are various engine and vehicle operating parameters such as: integrated time-temperature history of engine oil, speed-time histograms of engine or transmission, engine load vs. time data, and oil contamination criteria.
Various electronic instruments have been developed to differentially weight these parameters, sum them according to a particular transfer function, establish permissible limits for the measured variables, and then alert the vehicle operator if these limits are exceeded.
More cost effective approaches have been developed wherein the service interval (which in automotive use is primarily the oil change interval) is determined by modeling (mathematically) the degradation of motor oil. The principal parameters affecting the degradation of motor oil are, time at temperature (t.e.sup.kt) and the "contamination" factors. An exemplary system is disclosed in U.S. Pat. No. 4,007,629 in the name of Peter A. Hochestein.
Even the best service interval devices, electronic or mechanical, are not able to accurately determine the critically important lubricity of oil; either in an engine or automatic transmission. Contamination of engine oil by water soluble acids, water, particulate matter (dust or carbon) and oil degradation by-products severely affects the ability of the motor oil to lubricate, cool, and protect critical engine parts. Contamination of autommatic transmission fluid by particulates, water, and other foreign matter, can also affect the mechanical function of mechanism, and in fact, cause damage to the precision mechanical valve body components in the hudraulic servo controls.
While the actual process of oil or transmission fluid degradation is different in an engine (heat, blow-by, etc) and in an automatic transmission (primarily thermal decomposition), the requirement to continually audit the fluid medium is the same.
Mineral, petroleum based motor oils, synthetic ester motor oils, and hydraulic fluid experimental data suggest that while the initial resistance of these oils is variable, the general behavior with temperature is similar, and follows an exponential curve.
Contamination of these fluids in use, changes the initial resistivity at a given temperature. However, the behavior relating to decreases in resistivity with increasing temperature is still valid.
Samples of used motor oil exhibiting decreases in resistivity as a function of usage confirm that contaminants or degradation of the oil base itself slowly increase the conductivity of the fluid.
Naturally, the effect of temperature on the resistivity of the fluid must be taken into account, if a true measure of resistivity change with use is desired. Factoring out the apparent change in resistivity due to temperature is critical because the slope of temperature dependence is greater than the slope of use (and contamination) dependence over the typical engine oil operating temperature range.